FUNCTIONS OF THE TONGUE.- The phy siology of the tongue, like its organisation, is double, all its functions being referable either to those sensory or muscular endow ments which it possesses in so remarkable a degree. Naturally, these are intimately associated, its sensibility being necessary for the direction of its muscular action, and its movements necessary for the perfection of its sensibility; a systematic consideration of them, however, necessitates their separation, and those functions that are referable to the tongue as an organ of sensibility, have already been treated of in the articles TASTE and Toucu, to which the reader is referred ; it only remains for me to consider those that it possesses as an organ of motion.
These are prehension, mastication, insaliva tion, deglutition, speech, and one or two unim portant and non-essential offices in which it is engaged, which may be called the accidents of its physiology, as despuition or spitting, whistling, &c. Of these the four first-men tioned belong to the tongue as an organ of digestion ; they are, in fact, the first four stages of that process; all four exist in all mammalia, the first and the last in all vertebrate ; speech and the other non-digestive motor functions are peculiar to man.
Prehension. — The tongue is not, properly speaking, in man, an organ for the prehension of soliefood, that office being performed by the hand, for which the opponent arrange ment of thumb and fingers eminently fits it, so that the human tongue has not those ad ditional qualifications which we find in other animals to adapt it for an organ of pre hension. And this, I may remark, is an in stance of a very general law—that the ascent in the animal scale is not a passage from animals with simple organs to animals with complex organs, but from simple individuals with organs of complex function, to complex individuals with organs of simple function, the addition as we ascend being, not of functions, but of parts to discharge those functions, and the advantage gained, not another thing done, but the same thing done better. Thus in man, instead of having one office more, the tongue has one office less than in many animals below him ; and the delicate and extended prehen sion supplied by his hands, diminishes by one item the complexity of function, and, there fore, of organisation of his tongue. So that in judging of the elevation of animals by their individual organs, supposing such a method to be admissible, we must not look to complex ity of structure of those organs, but to the perfection of the resulting function. But to return.
In the prehension of liquids, or suction, the tongue in man is engaged ; constituting a movable wall of the oral cavity, it acts as a piston, and draws the liquid into the mouth by the formation of a temporary vacuum.
Bichat enumerates three methods of the pre hension of liquids, by suction, by drinking from a vessel, and by infusion into the throat; in the first two the tongue is concerned ; in the last, which seems to me hardly to deserve the name of prehension, it is not. In suction, which is peculiar to the infant, the nipple is seized by the lips, which are compressed around it by the orbicularis oris; the velum palati is elevated so as to close the posterior nares; the tongue forms, by the contraction of the middle fibres of the genio-glossus, which depress its centre, a longitudinal channel, which receives the nipple, and transmits the milk to the pharynx as long as a vacuum con tinues to be formed. In drinking liquids from a vessel the tongue forms a channel for its transmission, but it flows down to the pharynx by its own proper gravity.
Mastication.—As far as relates to the tongue, which here, however, is only subsidiary to the teeth, the mechanism of mastication may be divided into three stages :—first, that of placing the food in an advantageous position with regard to the teeth; secondly, affecting the position of the food in a definite manner when under the action of the teeth ; thirdly, collecting the scattered portions of masticated aliment prior to deglutition. Immediately on the introduction of a morsel of food into the mouth, either bitten by the incisors or other wise, it is at once transferred to the molars, so that it shall project beyond them, outwards, against the cheek ; the cheek is then pushed against it by the action of the buccinator, and the food is slowly driven across the teeth, which are rhythmically opened and closed, the tongue at the same time pushing moderately against it on the inside and so regulating the movement imparted to it by the cheek. Thus we see that the food under mastication is sub jected to an equable and regulated motion ; that it is placed, as it were, between two movable walls, and that by the even lateral movements of these walls, and the rhythmical vertical action of the teeth, its perfect masti cation is secured. As soon as the cheek has pushed it inwards as far as it can, an interval in the rhythmical closure of the teeth takes place, and the tongue restores it to its former position, again to be pushed inwards, and so on. The equable mastication of the food is secured much in the same way as the even motion of a rod of timber, under the blade of a circular saw, secures the cutting off of pieces of equal thickness. Any one who watches himself whilst eating will at once observe the sets of rhythmical action, interrupted by short interval, in which the food is restored to its necessary position between the teeth. The third stage, that of collecting the food from all parts of the mouth, admits of no particular description.